1. Field of the Invention
The invention relates generally to photographic printers having automatic exposure control and more particularly to such printers which base exposure determinations, at least in part, on densities measured for the original as a whole for a set of three primary colors.
2. Description Relative to the Prior Art
Photographic printers incorporating automatic exposure control are well known and produce the bulk of all photographic prints. Because rather small changes in printing times among the primary colors (generally blue, green and red) can have marked effects on final print quality, exposure determination is an operation which is critical to the performance of such printers.
Printers have over the years become more sophisticated with respect to exposure determination and, with this increased sophistication, a greater burden in compensating for picture-taking errors has been undertaken at the printing step in the photographic sequence, i.e., the sequence from film to final print. For example, the most common type of camera (fixed aperture, fixed shutter speed) has no exposure control apparatus and modern automatic printers, in effect, provide these cameras with exposure control by compensating for exposure inaccuracies over a moderate range.
Other situations where printers serve to compensate for adverse influences on picture taking results include illuminant variations (e.g. where the illuminant, say daylight does not correspond to the color balance of the film, say film balanced for tungsten lighting) and out of date film which tends to cause an improper color balance in the final print.
While expanding the role of the printer in correcting for exposure errors can prove highly beneficial, the problem of determining a "best" set of printing exposure times for three primary colors does not yield to easy solution. Most printers, in assessing transparencies for corrective action, measure average transmission densities (commonly called large area transmission densities--LATDs) for three primary colors--say, red, blue and green--and various techniques have been developed for determining exposure times based on these LATD measurements. In early printers, corrective action for any given primary color was based on the measured LATD for that particular color. The advent of the "matrix" printer (see, for example, U.S. Pat. No. 3,120,782) represented a substantial improvement in exposure determination in that interaction effects among the colors could be taken into account; e.g. the interaction resulting because the red density monitor filter blocks green light to a different degree than does the red sensitive emulsion of the printing paper.
[The logarithm of each color exposure for matrix printing is based, at least in part, on a linear function of three different color densities. For further background, see SPSE Handbook of Photographic Science and Engineering, Section 7.10, "Color Printing".]
If a high correction level is maintained with a matrix printer, however, unacceptable prints occur for a significant percentage of scenes. Indeed, "subject failure" is a term which has been used in categorizing situations where the scene content chosen by the photographer "tricks" the exposure control device of the printer into determining an undesirable correction.
Subject failure typically occurs because matrix printers operate based on the assumption that exposures should be highly corrected in a direction for achieving a near neutral print (see U.S. Pat. No. 2,571,697). Such correction, in general , compensates for color-distorting illuminant effects or film aging effects quite satisfactorily and, hence, high correction levels are desirable for transparencies exhibiting those characteristics. High corrections toward neutral average density are undesirable, however, for that population of scenes where one color intentionally predominates, e.g. scenes which have large areas of green grass or blue sky.
To reconcile these conflicting correction requirements, various discriminant parameters have been used which, for preselected regions in color space serve to trigger the introduction of an augmenting exposure which counteracts the influence of the correction matrix (see e.g. U.S. Pat. Nos. 3,502,410 and 3,697,174). Such augmenting exposures may vary quantitatively in relation to location within a region but typically have a fixed color ratio for each region; hence, some hue change, i.e., "color rotation" typically occurs in counteracting the correction. It is also known to direct each augmenting exposure radially from neutral density to avoid color rotation effects. To provide such a secondary correction radially away from neutral density, however, a vector direction must first be defined and then a scaling factor is applied to adjust the magnitude of the augmenting exposure which counteracts the primary correction.
Another approach for changing printer correction utilizes two matrices one for high correction and one for low correction. In U.S. Pat. No. 3,653,759 apparatus is described for switching from a high correction matrix to a low correction matrix for achieving intermediate levels of correction. U.S. Pat. No. 3,697,174 describes apparatus which utilizes a high correction matrix but selectively compresses the times between filter insertions to provide an adjustment to correction level.